There are always differences with every home you install a system on. We recently went to a home where the homeowner and a 7 year old, 4 ton system. He complained that it never conditioned the home properly and it ran constantly.
I showed him how the duct work was undersized and he was not even getting 2 tons out of the unit. He was surprised that other companies had not mentioned this problem. We got the job and I redesigned the system with 3 return airs and over sized supplies. Our Hers rater said he had not seen a system run as well as ours in a long time. It is over performing and running extremely efficient. The client was very pleased and has recommended us to other friends as well.